Shadow Creatures are hostile mobs that begin to attack player upon reaching 30 sanity points or lower. They will spawn once the player's sanity levels are below 82.5% and they will gradually become more visible as the player's sanity level drops further. They are visible only to the player and cannot be examined.

When the player is insane, up to three of these creatures will spawn around the player and attack. Upon being killed, they will drop Nightmare Fuel, used for crafting several Magic items.

The sanity meter will drop by being near aggressive Shadow Creatures because of their insanity aura, but it will rise by at least 15 points after killing one.

Shadow Creatures also appear in Ruins, spawning from Nightmare Lights and Nightmare Fissures during Nightmare Phase in the Nightmare Cycle. They'll be tangible and hostile to the player regardless of sanity and will pursue the player until the cycle is at the calm stage again. When a Shadow Creature is spawned from a Nightmare Light, killing it will not increase your sanity.

Originally believed to be the name of all the Shadow Creatures, the Crawling Horror is now known to be just one type.

It is a large tick-like creature, with many legs, and is the most common of the Shadow Creatures.

While not aggressive (above 15% Sanity), Crawling Horrors will wander around, move away from the player when the player gets too close, and occasionally teleport around the player's vicinity. They can disturb the webbing around Spider Dens, causing inhabiting Spiders to emerge and investigate the cause. They also can prevent items and the ground underneath them from being clicked on, even when they're not entirely solid.

Crawling Horrors turn aggressive when the player's sanity is 15% or below (18 for Willow, 15 to 45 for WX-78, 37.5 for Wickerbottom, 22.5 for Wes and 30 for all other characters). Walking near these will decrease Sanity, and they will restore 15 Sanity and drop Nightmare Fuel when killed.

A version of the Crawling Horror called Crawling Nightmares can be encountered in the Ruins. This version is able to attack even when the player is sane, but they will only spawn from Nightmare Lights when they are completely open. When killed, it will not restore sanity.

With normal damage modifier characters, they take 9 hits with a Spear; 8 with a Bat Bat; 6 with a Tentacle Spike, Ham Bat, and Thulecite Club; and 5 with Dark Sword to be killed.